White light emitting LEDs (“white LEDs”) include one or more photoluminescence materials (e.g., phosphor materials), which absorb a portion of excitation radiation emitted by the LED and re-emit radiation of a different color (wavelength). Typically, the LED chip or die generates blue light and the phosphor(s) absorbs a percentage of the blue light and re-emits yellow light or a combination of green and red light, green and yellow light, green and orange or yellow and red light. The portion of the blue light generated by the LED that is not absorbed by the phosphor combined with the light emitted by the phosphor provides light which appears to the human eye as being nearly white in color. Due to their long operating life expectancy (>50,000 hours) and high luminous efficacy (100 lm/W and higher) white LEDs are rapidly being used to replace conventional fluorescent, compact fluorescent and incandescent lamps.
Recently, LED-filament lamps have been developed comprising LED-filaments whose visual appearance closely resemble the filament of a traditional incandescent lamp. The LED-filaments, which are typically about two inches (50 mm) long, comprise COG (Chip-On-Glass) devices having a plurality of low-power LED chips mounted on a light-transmissive glass substrate. The LED-filaments are encased in a phosphor-impregnated encapsulant, such as silicone.
One perceived shortcoming or characteristic of white LEDs, LED-filaments and fluorescent lamps, compared with traditional incandescent bulbs, is that as they are dimmed the color temperature (CCT) of their light emission does not change; that is, it remains substantially constant. By contrast—for an incandescent lamp—the CCT can change from a warm white (2700 K) at full power to a warm orange glow (900 K) when fully dimmed; a range of 1800K. For many applications, such a characteristic would be highly desirable for LED-filaments and white LEDs. It is, therefore, an object of the present invention to provide a solid-state white light emitting device that at least in part addresses the limitations of known devices and whose color temperature of emitted light decreases on dimming, thus resembling, at least in part, a conventional incandescent light bulb.